This Program Project is devoted to the detailed analysis of structure-function correlations in the first auditory area (AI) of the cerebral cortex. The ultimate aim is to reveal the determinants of binaural processing in the auditory cortex. For the purposes of description, the Program consists of six project areas and a central core. Virtually all the projects, however, involve strong interactions, including the sharing of animal material and equipment and they are mainly distinguished by the nature of the experimental approaches used, each in the hands of an investigator highly experienced in those techniques. Thus: (i) The development of AI will be analyzed in single unit studies, in histochemical and immunocytochemical studies, in receptor binding studies and by in situ hybridization to detect mRNAs. (ii) The potential plasticity of AI in response to cochlear damage or inactivation will be investigated in both developing and mature animals by the same range of methods. (iii) The nature of intracortical processing in AI will be examined in a detailed physiological study of binaural interactions, in a study of laminar differences in the properties of individual neurons and in a combined study of intracellularly recorded and injected neurons and immunocytochemically labeled neurons. (iv) The detailed circuitry of input-output relations in AI will be defined in physiological, anatomical and chemical terms using extra- and intracellular recording of defined classes of neurons and afferent fibers, electron microscopy of identified cells and axons, immunocytochemistry of transmitter defined cells and pathways, and receptor binding. In these investigations a particular animal model appropriate for the immediate specific aims has been chosen and the experiments will focus on development and plasticity in AI of the rat and ferret, physiology and chemical and molecular anatomy of AI in the cat and transmitter plasticity in AI of the adult monkey.